Pollen quality matters. Plants generally like to outcross pollen, so external pollen is better than a plant’s own pollen. If seeds don’t travel far from a plant, then neighbours are likely to be close relatives. So pollen from distant plants performs better than closer plants. But go too far, and the genetic differences become too great, and pollen becomes less and less fruitful. Presumably, there’s an Optimal Outcrossing Distance, but how far away is that? Bronwyn Ayre and colleagues have been experimenting to find the Optimal Outcrossing Distance of Anigozanthos manglesii.
A, manglesii, the Red and Green Kangaroo Paw, is a plant found in Southwest Australia, about 30cm to 80cm tall (between a foot high to two and a half feet). At the top is a striking red and green flower. The flower is distinctive and aimed at birds, which is how Ayre got involved: “This work is part of my PhD, which is looking at the critical role birds play in the pollination of the Red and Green Kangaroo Paw. Here, in the South West of Western Australia, we have a higher percentage of bird and mammal pollinated species than anywhere else (15% of our flowering plants). This, unfortunately, includes 40% of plant species threatened with extinction. We don’t know why bird and mammal pollinated plants have a higher chance of being threatened – we need more information!”
Survival improves if a plant produces more seeds. Fortunately, Ayre said, there is plenty of work that has been done on this in other plants. “We’ve built on an experiment that’s been done many times, using hand pollination to cross plants at varying distances, to determine if there’s one distance that’s the “best”. While past experiments have near exclusively done this using donor pollen from a single plant, we’ve also provided pollen from multiple plants at once and used genetics to determine which donor plant fathers the most seeds. We found that near-neighbour pollen had an advantage over pollen that came from more distant plants, fathering more seeds in multi-donor crosses, and resulting in higher seed set in single donor crosses.”
The experiment uses two approaches to look at the genetic effects. One is looking at the genes, but many earlier studies didn’t do this. Instead, they looked at spatial distance. Ayre explained: “Spatial distance is a different way of looking at genetic distance- sort of a short cut.”
“Genetics is expensive, time-consuming, and the genetic tools we have today really weren’t available ten, twenty years ago. This means a lot of studies do hand-pollination crosses between plants at different spatial distances, and we assume that the further apart two individual plants are the less related they are.”
“While that isn’t a bad assumption, genetics just gives us a more detailed look at what’s going on. In this study, we see that beyond ~3m, there’s no evidence that the further away a plant is, the less related it is. To put that another way, there’s no evidence that two plants 10m apart from each other are any more or less related than two plants 30m apart.”
Ayre said that Anigozanthos manglesii was a particularly good plant for this kind of experiment. “While the Red and Green Kangaroo Paw isn’t a threatened species itself, it’s a great model species to work with. For hand-pollination experiments they’re great- the flowers are large, sturdy and produce a lot of pollen. The only problem I had was with some of the local bush crickets- they’d occasionally find their way into the bags I put around my plants (to stop any birds or bees getting access to my experimental flowers) and eat all the pollen overnight.”
The hand pollination experiments found that there was indeed inbreeding depression when pollen was transferred between nearest neighbours. This effect was evident up to about three metres, after which the pollen was at the optimal distance. This means the optimal outcrossing distance is within the local population of plants.
The authors compared this with other studies and found that bird-pollinated species were more likely to have a within-population optimal outcrossing distance. This came as a surprise, as Ayre said: “It is definitely not what I was expecting- we generally assume that bird-pollination results in pollen being moved larger distances than insect-pollination. So, I thought that would mean that insect-pollinated plants would have a within-population optimal outcrossing distance more often than bird-pollinated plants.”
“It’s hard to draw a strong conclusion about the pattern we see, as there hasn’t been a lot of work done in this area. We’ve only compared 22 species in this paper. It may be that if (or hopefully when!) these sorts of experiments are run in other species, the trend we see here will change.”
While the results are clear, the reason why isn’t so obvious Ayre said: “As to the why? The simple answer is that we don’t really know. It may be that a combination of short-distance seed dispersal and wide-scale pollen dispersal is important. Short distance seed dispersal would mean that plants within a couple of metres are closely related. This means pollen dispersal across a relatively short distance will result in crosses between unrelated plants.”
The authors note that single-donor experiments are not realistic when compared to the natural world. The article shows how multi-donor experiments are possible for finding optimal outcrossing distances. Ayre sees this as an opportunity for people to do further experiments: “I would love to see more studies using genetics, doing multi-donor experiments, and looking closely at non-bee pollinated plants. While genetics is still expensive, prices are coming down, and it really gives us extra insight into what’s happening.”
Ayre, B. M., Roberts, D. G., Phillips, R. D., Hopper, S. D., & Krauss, S. L. (2019). Near-neighbour optimal outcrossing in the bird-pollinated Anigozanthosmanglesii. Annals of Botany. https://doi.org/10.1093/aob/mcz091